fix(a11y): improve dynamic tool config field feedback and navigation

[saurabhhhcodes]

Summary

• improve ToolConfig accessibility wiring for dynamic fields
• focus the first invalid field before blocking scan start
• preserve help text alongside error messaging and tighten button semantics

Testing

• git diff --check
• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/vitest run testing/unit/pages/ToolConfigDynamic.test.tsx testing/unit/pages/ToolConfigRouting.test.tsx (blocked locally by macOS Rollup native-signature loading issue in this environment; changes still include focused test coverage updates)

Closes #238

[saurabhhhcodes]

Opened this one for issue #238 with focused accessibility updates in ToolConfig. The change set stays local to dynamic field feedback, invalid-field focus, and route/button accessibility wiring.

[saurabhhhcodes]

Pushed a follow-up accessibility fix for the failing frontend checks: the scan button now keeps invalid-field focus behavior available via aria-disabled instead of native disablement during validation errors, and the duplicate attention copy in the sidebar status was removed. Local frontend typecheck and ToolConfigDynamic.test.tsx now pass on the updated branch.